Protege Cam thread.

[2.0t03speed]

it's different on a turbo motor w/ a small turbo like ours that produces high amounts of back pressure you wan't as little overlap as possible to help prevent reversion. That's why the twiggy's iirc have a slightly negative overlap.

 

[Alejo_NIN]

Overlap is when both intake and exhaust valves are open. You want a little overlap so that the new air coming in to charge the cylinder with A/F can push out the rest of the burnt combustion from the previous cycle. More overlap from my experience, which is limited, will increase bottom end power by increasing the amount of A/F entering the cylinder for the compression stroke (filling the chamber) by opening earlier in the exhaust stroke . The problem is that while you are clearing the chamber of burnt air you are also sending your new mixture into your exhaust where it is wasted. This will reduce mileage and efficiency of the motor. There is a lot going on all at once but the most critical part is overlap IMO. Too much and your are wasting gas and burning CATS up, too little and you can starve the engine creating lean situations that will rob power. There is a lot of geometry involved for the entire process. Each "stroke" lasts one half crankshaft revolution or 180 crankshaft degrees. Since the camshaft turns at half of the speed of the crank, the power stroke only sees one fourth of a turn of the cam, or 90 camshaft degrees.

Lift can be adjusted by not only by cam profile. Tappet lift x rocker ratio= valve lift.

I hope this gives you a little more to consider... I know when I was reading up on the cams and types I could not grasp the idea completely but once I looked at the head to start work a light bulb went off and I felt the beginning stages of comprehension. Now I believe I have gone and forgotten quite a bit. I had a few articles stored on my home PC I can send your way... if your interested that is

thanks for clearing that up, sorta.
this subject is something that most people don't even want to think about but i believe is one of the steps that will yield the most performance.
people worry too much about other aspects and forget about the basics.

ok, so overlap keeps the intake and exhaust valves open longer.
so on a boosted car, you'd want to keep them open longer thus allowing more air to come in.
now what you call duration is how long, or for how many degrees the valves stay open and closed.
being that a cam is ideally 360 degrees, the number you use to descrive duration is actually part of those 360 degrees, i.e. on j-spec intake cam, the valve stays open for 241 degrees opening 5 degrees BTDC and closing 56 degrees ATDC....or is it that this specific cam leaves the valve closed for 241 degrees and open it for 61 degrees starting 5 BTDC and ending 56 ATDC?
if so, 241 degrees plus 61 degrees equals 302 degrees....where do the other 58 degrees go?

 

[Alejo_NIN]

i think i got it.
the 241 degrees come from the "LOBE" and not the circumerence of the CAM
with that, yes, the imaginary circle at the end of the "LOBE" will be about 241 degrees in contact with the valve stem.
Hence why it also mentions how long it stays closed, for about 197 degree, that's part of the circumference of the actual CAM and not the lobe...right?

 

[2.0t03speed]

here is one of lord worms older post's that i found about choosing cams.

theres a little more to it than that. ramp rate is at least as important (how quickly the valves are opened, and closed, as opposed to how long they are open for (duration) and how far they are opened (lift)). It is possible to have a big cam from an advertised duration/gross lift point of view, that gets killed by a cam with less advertised duration/gross lift but opens the valves more rapidly... it is very difficult to say a cam is good/bad/better than another based on one set of measurements. Unfortunately, most of the time thats all you get....

Also the relationship between lift and duration is important....as well as overlap, exhaust and intake cam advance etc etc.

turbo engines (especially not-so-worked engines....just about anything thats not a race motor) tend to benefit from more lift, where as NA motors tend to enjoy long durations. Of course both are important in either application.

Also, unless you are running a completely insane race motor, low, or no overlap is important for a turbo setup to prevent bleeding boost off through an open exhaust valve on the intake stroke (i believe theres actually negative-overlap on the twiggy cams for FI from memory..could be wrong though).... where as NA will enjoy a larger overlap to assist with scavenging and high RPM breathability.

Then theres the issue of dynamic compression with large cams and so on and so forth...

my point is - its not just a case of looking at the advertised lift and duration in isolation - cams are arguably the most complicated part of the motor to get right.

 

[Alejo_NIN]

here is one of lord worms older post's that i found about choosing cams.

alright
so this clarifies some question in regards to turbo setups, which is what i am interested in.
it also shows i was kinda right on assuming that on a turbo setup you'd want as little overlap as possible even to the point of having the exhuast cam closing first as to prevent boost leak?

it is ok if you'd think i'm starting to get annoyed...but i am trying really hard to understand this.

 

[gangstagangsta]

i think i got it.
the 241 degrees come from the "LOBE" and not the circumerence of the CAM
with that, yes, the imaginary circle at the end of the "LOBE" will be about 241 degrees in contact with the valve stem.
Hence why it also mentions how long it stays closed, for about 197 degree, that's part of the circumference of the actual CAM and not the lobe...right?

The duration degree is for each stroke of the motor. You can see in the graph how the crank will spin twice as the cams make their single full rotation. So each complete rotation of the crank is 360* but it is only lifting one part of the system intake or exhaust.

 

[VIPMSP]

sooo what cams should i get w/ my K1 rods/wiseco piston/steedspeed/GT28RS combo?

are the SR motorsport ones any good?

http://www.srmotorsports.com/Mazda_Protege_Performance_Engine_Parts.html

 

[gangstagangsta]

No hard numbers to do the geometry crunch but increasing lift and duration is good. Those are the CAM gears I am using in my build but I went with the CorkSport cams.

 

[2.0t03speed]

the sr cams are just jdm reps they are the same cam corksport sells

 

[VIPMSP]

lemme get them twiggys...


... but really if you come across another set, notify me! that video sounds sweet

 

[2.0t03speed]

will do bro i had to do some digging to get these. The last set i saw for sale were twilights(twiggy) og cams. Before that is was the set i sold a few years back.

 

[Installshield 2]

I totally missed this a few months ago..

On the note of increasing lift...lobe height effects that, as well as valve stem length/tappet thickness...simply adding thicker shims does in fact increase lift, but also decreases lash clearance...and too little of that, problems quickly develop...so other than modifications to the cam lobes, you're pretty limited in terms of the valves and tappets...for max output purposes the generally accepted practice is to have the least amount of lash possible...too much lash is just wasting lift...too little lash will burn everything up eventually...but being on the tight side of the recommended specs is ideal for max lift...

Just finally got my set of NA twiggys installed (long story)...no joke, simply ridiculous fixed overlap...it'll take A LOT of revs to choke them up...and i had to make some pretty aggressive adjustments just to not stall with an MP3 ecu...

 

[jbrewer4639]

will do bro i had to do some digging to get these. The last set i saw for sale were twilights(twiggy) og cams. Before that is was the set i sold a few years back.

Yeah, those are sitting in my closet.

 

[jbrewer4639]

I forgot to post this awhile back for everyone wondering about the twiggys overlap read post #714